The invention pertains to memory devices, such as, for example, dynamic random access memory (DRAM) devices; and in particular aspects pertains to electronic systems comprising memory devices. The memory devices can utilize thin film transistor (TFT) constructions.
SOI technology differs from traditional bulk semiconductor technologies in that the active semiconductor material of SOI technologies is typically much thinner than that utilized in bulk technologies. The active semiconductor material of SOI technologies will typically be formed as a thin film over an insulating material (typically oxide), with exemplary thicknesses of the semiconductor film being less than or equal to 2000 xc3x85. In contrast, bulk semiconductor material will typically have a thickness of at least about 200 microns. The thin semiconductor of SOI technology can allow higher performance and lower power consumption to be achieved in integrated circuits than can be achieved with similar circuits utilizing bulk materials.
An exemplary integrated circuit device that can be formed utilizing SOI technologies is a so-called thin film transistor (TFT), with the term xe2x80x9cthin filmxe2x80x9d referring to the thin semiconductor film of the SOI construction. In particular aspects, the semiconductor material of the SOI construction can be silicon, and in such aspects the TFTs can be fabricated using recrystallized amorphous silicon or polycrystalline silicon. The silicon can be supported by an electrically insulative material (such as silicon dioxide), which in turn is supported by an appropriate substrate. Exemplary substrate materials include glass, bulk silicon and metal-oxides (such as, for example, Al2O3). If the semiconductor material comprises silicon, the term SOI is occasionally utilized to refer to a silicon-on-insulator construction, rather than the more general concept of a semiconductor-on-insulator construction. However, it is to be understood that in the context of this disclosure the term SOI refers to semiconductor-on-insulator constructions. Accordingly, the semiconductor material of an SOI construction referred to in the context of this disclosure can comprise other semiconductive materials in addition to, or alternatively to, silicon; including, for example, germanium.
A problem associated with conventional TFT constructions is that grain boundaries and defects can limit carrier mobilities. Accordingly, carrier mobilities are frequently nearly an order of magnitude lower than they would be in bulk semiconductor devices. High voltage (and therefore high power consumption), and large areas are utilized for the TFTs, and the TFTs exhibit limited performance. TFTs thus have limited commercial application and currently are utilized primarily for large area electronics.
Various efforts have been made to improve carrier mobility of TFTs. Some improvement is obtained for devices in which silicon is the semiconductor material by utilizing a thermal anneal for grain growth following silicon ion implantation and hydrogen passivation of grain boundaries (see, for example, Yamauchi, N. et al., xe2x80x9cDrastically Improved Performance in Poly-Si TFTs with Channel Dimensions Comparable to Grain Sizexe2x80x9d, IEDM Tech. Digest, 1989, pp. 353-356). Improvements have also been made in devices in which a combination of silicon and germanium is the semiconductor material by optimizing the germanium and hydrogen content of silicon/germanium films (see, for example, King, T. J. et al, xe2x80x9cA Low-Temperature ( less than =550xc2x0 C.) Silicon-Germanium MOS TFT Technology for Large-Area Electronicsxe2x80x9d, IEDM Tech. Digest, 1991, pp. 567-570).
Investigations have shown that nucleation, direction of solidification, and grain growth of silicon crystals can be controlled selectively and preferentially by excimer laser annealing, as well as by lateral scanning continuous wave laser irradiation/anneal for recrystallization (see, for example, Kuriyama, H. et al., xe2x80x9cHigh Mobility Poly-Si TFT by a New Excimer Laser Annealing Method for Large Area Electronicsxe2x80x9d, IEDM Tech. Digest, 1991, pp. 563-566; Jeon, J. H. et al., xe2x80x9cA New Poly-Si TFT with Selectively Doped Channel Fabricated by Novel Excimer Laser Annealingxe2x80x9d, IEDM Tech. Digest, 2000, pp. 213-216; Kim, C. H. et al., xe2x80x9cA New High-Performance Poly-Si TFT by Simple Excimer Laser Annealing on Selectively Floating a Si Layerxe2x80x9d, IEDM Tech. Digest, 2001, pp. 753-756; Hara, A. et al, xe2x80x9cSelective Single-Crystalline-Silicon Growth at the Pre-Defined Active Regions of TFTs on a Glass by a Scanning CW Layer Irradiationxe2x80x9d, IEDM Tech. Digest, 2000, pp. 209-212; and Hara, A. et al., xe2x80x9cHigh Performance Poly-Si TFTs on a Glass by a Stable Scanning CW Laser Lateral Crystallizationxe2x80x9d, IEDM Tech. Digest, 2001, pp. 747-750). Such techniques have allowed relatively defect-free large crystals to be grown, with resulting TFTs shown to exhibit carrier mobility over 300 cm2/V-second.
Another technique which has shown promise for improving carrier mobility is metal-induced lateral recrystallization (MILC), which can be utilized in conjunction with an appropriate high temperature anneal (see, for example, Jagar, S. et al., xe2x80x9cSingle Grain TFT with SOI CMOS Performance Formed by Metal-Induced-Lateral-Crystallizationxe2x80x9d, IEDM Tech. Digest, 1999, p. 293-296; and Gu, J. et al., xe2x80x9cHigh Performance Sub-100 nm Si TFT by Pattern-Controlled Crystallization of Thin Channel Layer and High Temperature Annealingxe2x80x9d, DRC Conference Digest, 2002, pp. 49-50). A suitable post-recrystallization anneal for improving the film quality within silicon recrystallized by MILC is accomplished by exposing recrystallized material to a temperature of from about 850xc2x0 C. to about 900xc2x0 C. under an inert ambient (with a suitable ambient comprising, for example, N2). MILC can allow nearly single crystal silicon grains to be formed in predefined amorphous-silicon islands for device channel regions. Nickel-induced-lateral-recrystallization can allow device properties to approach those of single crystal silicon.
The carrier mobility of a transistor channel region can be significantly enhanced if the channel region is made of a semiconductor material having a strained crystalline lattice (such as, for example, a silicon/germanium material having a strained lattice, or a silicon material having a strained lattice) formed over a semiconductor material having a relaxed lattice (such as, for example, a silicon/germanium material having a relaxed crystalline lattice). (See, for example, Rim, K. et al., xe2x80x9cStrained Si NMOSFETs for High Performance CMOS. Technologyxe2x80x9d, VLSI Tech. Digest, 2001, p. 59-60; Cheng, Z. et al., xe2x80x9cSiGe-On-Insulator (SGOI) Substrate Preparation and MOSFET Fabrication for Electron Mobility Evaluationxe2x80x9d 2001 IEEE SOI Conference Digest, October 2001, pp. 13-14; Huang, L. J. et al., xe2x80x9cCarrier Mobility Enhancement in Strained Si-on-Insulator Fabricated by Wafer Bondingxe2x80x9d, VLSI Tech. Digest, 2001, pp. 57-58; and Mizuno, T. et al., xe2x80x9cHigh Performance CMOS Operation of Strained-SOI MOSFETs Using Thin Film SiGe-on-Insulator Substratexe2x80x9d, VLSI Tech. Digest, 2002, p. 106-107.)
The terms xe2x80x9crelaxed crystalline latticexe2x80x9d and xe2x80x9cstrained crystalline latticexe2x80x9d are utilized to refer to crystalline lattices which are within a defined lattice configuration for the semiconductor material, or perturbed from the defined lattice configuration, respectively. In applications in which the relaxed lattice material comprises silicon/germanium having a germanium concentration of from 10% to 60%, mobility enhancements of 110% for electrons and 60-80% for holes can be accomplished by utilizing a strained lattice material in combination with the relaxed lattice material (see for example, Rim, K. et al., xe2x80x9cCharacteristics and Device Design of Sub-100 nm Strained SiN and PMOSFETsxe2x80x9d, VLSI Tech. Digest, 2002, 00. 98-99; and Huang, L. J. et al., xe2x80x9cCarrier Mobility Enhancement in Strained Si-on-Insulator Fabricated by Wafer Bondingxe2x80x9d, VLSI Tech. Digest, 2001, pp. 57-58).
Performance enhancements of standard field effect transistor devices are becoming limited with progressive lithographic scaling in conventional applications. Accordingly, strained-lattice-channeled-field effect transistors on relaxed silicon/germanium offers an opportunity to enhance device performance beyond that achieved through conventional lithographic scaling. IBM recently announced the world""s fastest communications chip following the approach of utilizing a strained crystalline lattice over a relaxed crystalline lattice (see, for example, xe2x80x9cIBM Builds World""s Fastest Communications Microchipxe2x80x9d, Reuters U.S. Company News, Feb. 25, 2002; and Markoff, J., xe2x80x9cIBM Circuits are Now Faster and Reduce Use of Powerxe2x80x9d, The New York Times, Feb. 25, 2002).
Although various techniques have been developed for substantially controlling nucleation and grain growth processes of semiconductor materials, grain orientation control is lacking. Further, the post-anneal treatment utilized in conjunction with MILC can be unsuitable in applications in which a low thermal budget is desired. Among the advantages of the invention described below is that such can allow substantial control of crystal grain orientation within a semiconductor material, while lowering thermal budget requirements relative to conventional methods. Additionally, the quality of the grown crystal formed from a semiconductor material can be improved relative to that of conventional methods.
In another aspect of the prior art, a common memory device is a DRAM comprising a combination of a transistor and a capacitor. A continuing goal of semiconductor device fabrication is to reduce a footprint associated with DRAM devices in order to conserve valuable semiconductor substrate real estate, and to contain fabrication costs. Fabrication of currently employed trench capacitor and/or stacked capacitor designs depend primarily on plate area and utilize complex processing adversely affecting device yield. Numerous configurations of capacitor constructions have been proposed in an effort to obtain desired capacitance within a relatively small footprint. There remains room for improvement in capacitor design, and it would be desirable to develop better capacitor designs.
In one aspect, the invention includes a memory device comprising a transistor and a capacitor. The transistor can be a TFT. The transistor comprises a source/drain region, and the capacitor comprises a storage node in electrical connection with the source/drain region. The capacitor also comprises a horizontally-extending reference plate which splits into at least two prongs. Additionally, the capacitor includes a dielectric material surrounding the prongs of the reference plate. The storage node surrounds a majority of a lateral periphery of the prongs, and is separated from the reference plate prongs by at least the dielectric material. The utilization of the prongs in combination with surrounding a large amount of the lateral periphery of the reference plate with the storage node allows both area and perimeter capacitance to be advantageously utilized in exemplary devices of the present invention.
In particular aspects, the invention includes a memory device comprising a transistor and a capacitor, with the transistor having a gate proximate a crystalline layer. The crystalline layer can be less than or equal to 2,000 xc3x85 thick, and can comprise, consist essentially of, or consist of a silicon/germanium material, or an appropriately-doped silicon/germanium material. The transistor has an active region, and at least a portion of the active region is within the silicon/germanium material. The entirety of the active region within the silicon/germanium material is contained within a single crystal of the material. The crystalline silicon/germanium material can be over an insulative material, which in turn is over a base, and accordingly can be part of a semiconductor-on-insulator (SOI) construction. The base can comprise one or more of glass, aluminum oxide, silicon dioxide, metal, plastic, and a semiconductive-material wafer (such as, for example, a silicon wafer).
The invention can also include electronic systems comprising an array of memory cells, with one or more of the memory cells comprising one or more novel features of the types described within this disclosure for memory devices of the present invention.